With active use of portable electric products such as video cameras, cellular phones, portable PC or the like, secondary batteries mostly used as power sources of such products are becoming much more important.
Unlike a primary battery that is generally not chargeable, a secondary battery is chargeable and dischargeable. Such a secondary battery is being actively studied due to the development of high-tech products such as digital cameras, cellular phones, laptops, power tools, electric bicycles, electric vehicles, hybrid vehicles, large-capacity power storage devices or the like.
In particular, lithium secondary batteries are more actively used since they have high energy density per unit weight and allow rapid charging, in comparison to other kinds of secondary batteries such as lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries or the like.
A lithium secondary battery has an operating voltage of 3.6 V or above and is used as a power source of portable electronic devices or several lithium secondary batteries are connected in series or in parallel and used for high-power electric vehicles, hybrid vehicles, power tools, electric bicycles, power storages, UPS or the like.
Since a lithium secondary battery has an operating voltage three times higher than that of a nickel-cadmium battery or a nickel-metal hydride battery and also has an excellent energy density per unit weight, the use of lithium secondary batteries is rapidly growing.
A lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a polymer solid electrolyte depending on the kind of electrolyte. In addition, a lithium ion polymer battery may be classified into a solid-phase lithium ion polymer battery containing no electrolyte and a lithium ion polymer battery using a gel-type polymer electrolyte containing an electrolyte, depending on the kind of polymer solid electrolyte.
In most cases, a lithium ion battery using a liquid electrolyte uses a cylindrical or rectangular metal can as a container, which is sealed by welding. Since the can-type secondary battery using such a metal can has a fixed shape, the design of an electric product using such a power source is limited and is difficult to reduce in volume. Therefore, a pouch-type secondary battery where an electrode assembly and an electrolyte are put into a pouch package made of a film and then sealed has been developed and used.
However, since the lithium secondary battery may explode when overheated, it is very important to ensure its safety. The lithium secondary battery may overheat due to various factors. For example, when an overcurrent beyond a limit flows through the lithium secondary battery, the lithium secondary battery may overheat. If an overcurrent flows, the lithium secondary battery generates Joule's heat, which rapidly increases the internal temperature of the battery. In addition, the rapid increase of temperature decomposes the electrolyte, which causes thermal runaway and resultantly leads to explosion of the battery. An overcurrent occurs when a sharp metallic matter pierces the lithium secondary battery, when the insulation between a cathode and an anode is broken due to shrinkage of a separator interposed between the cathode and the anode, or when a rush current is applied to the battery due to abnormality of a charging circuit or load connected to the outside.
Therefore, in order to protect the battery against abnormal circumstances such as an overcurrent, the lithium secondary battery is coupled to a protective circuit. The protective circuit generally includes a fuse element for irreversibly disconnecting a wire through which a charge or discharge current flows, when an overcurrent occurs.
FIG. 1 is a circuit diagram for illustrating arrangements and operating mechanism of a fuse element of a protective circuit coupled to a battery pack including a lithium secondary battery.
As shown in FIG. 1, the protective circuit includes a fuse element 1 for protecting a battery pack when an overcurrent occurs, a sense resistor 2 for sensing an overcurrent, a microcontroller 3 for monitoring the occurrence of an overcurrent and operating the fuse element 1 when an overcurrent occurs, and a switch 4 for switching an operation current to flow to the fuse element 1.
The fuse element 1 is installed at a main line connected to the outermost terminal of the battery pack. The main line is a wire at which a charge current or discharge current flows. FIG. 1 shows that the fuse element 1 is installed at a high-voltage line (Pack+).
The fuse element 1 is a three-terminal element, in which two terminals are connected to the main line at which a charge or discharge current flows and one terminal is connected to the switch 4. In addition, the fuse element 1 includes a fuse 1a serially connected to the main line and fused to be cut at a specific temperature and a resistor 1b which applies heat to the fuse 1a. 
The microcontroller 3 monitors whether an overcurrent occurs or not by periodically detecting the voltage at both ends of the sense resistor 2, and when it is determined that an overcurrent occurs, the microcontroller 3 turns on the switch 4. Then, the current flowing through the main line bypasses to the fuse element 1 and is applied to the resistor 1b. Accordingly, Joule's heat generated by the resistor 1b is conducted to the fuse 1a to increase the temperature of the fuse 1a, and when the temperature of the fuse 1a reaches the fusing temperature, the fuse 1a fuses to be cut, thereby irreversibly disconnecting the main line. If the main line is disconnected, an overcurrent does not flow any more, which may solve problems caused by the overcurrent.
However, this technique has many problems. In other words, if the microcontroller 3 malfunctions, the switch 4 may not turn on even when an overcurrent occurs. In this case, a current does not flow to the resistor 1b of the fuse element 1, and therefore the fuse element 1 will not operate. Moreover, a space for disposing the fuse element 1 is separately needed in the protective circuit, and a program algorithm for controlling the operation of the fuse element 1 must be loaded in the microcontroller 3. Therefore, the space efficiency of the protective circuit deteriorates and the load of the microcontroller 3 increases. Accordingly, there is an increasing need for an improved technology to solve the outstanding problems.